Bispecific antibodies binding two different epitopes offer opportunities for increasing specificity, broadening potency, and utilizing novel mechanisms of action that cannot be achieved with a traditional monoclonal antibody. A variety of formats for bispecific antibodies that bind to two targets simultaneously have been reported. Cross-linking two different receptors using a bispecific antibody to inhibit a signaling pathway has shown utility in a number of applications (see, e.g., Jackman, et al., (2010) J. Biol. Chem. 285:20850-20859). Bispecific antibodies have also been used to neutralize two different receptors. In other approaches, bispecific antibodies have been used to recruit immune effector cells, where T-cell activation is achieved in proximity to tumor cells by the bispecific antibody which binds receptors simultaneously on the two different cell types (see Baeuerle, P. A., et al, (2009) Cancer Res 69(12):4941-4). Approaches developed to date have primarily involved bispecific antibodies that link the CD3 complex on T cells to a tumor-associated antigen. However in other examples, bispecific antibodies having one arm which binds CD16 (FcyRIIIa) and another which bound to an antigen of interest such as CD19 have been developed (see Kellner et al. (2011) Cancer Lett. 303(2): 128-139).
Natural killer (NK) cells are a subpopulation of lymphocytes that are involved in non-conventional immunity. NK cells provide an efficient immunosurveillance mechanism by which undesired cells such as tumor or virally-infected cells can be eliminated. Characteristics and biological properties of NK cells include the expression of surface antigens including CD16, CD56 and/or CD57, the absence of the alpha/beta or gamma/delta TCR complex on the cell surface; the ability to bind to and kill cells that fail to express “self” MHC/HLA antigens by the activation of specific cytolytic enzymes, the ability to kill tumor cells or other diseased cells that express a ligand for NK activating receptors, and the ability to release protein molecules called cytokines that stimulate or inhibit the immune response.
NK cell activity is regulated by a complex mechanism that involves both activating and inhibitory signals. Several distinct NK cell receptors have been identified that play an important role in the NK cell mediated recognition and killing of HLA Class I deficient target cells. One receptor, although not specific to NK cells, is FcγR3a (CD16) which is responsible for NK cell mediated ADCC. Another NK cell receptor is NKp46, a member of the Ig superfamily. It is specific to NK cells and its cross-linking, induced by specific mAbs, leads to a strong NK cell activation resulting in increased intracellular Ca++ levels, triggering of cytotoxicity, and lymphokine release. International patent publication number WO2005/105858 (Innate Pharma) discloses use of monospecific full-length IgG anti-NKp46 antibodies that bind Fcγ receptors for treating hematological malignancies that are Fcγ-positive. Fc gamma receptors on tumor cells (e.g. B cell malignancies) were proposed to interact with the Fc domain of the anti-NKp46 antibodies which bound NK cells, such that the activated NK cells are brought into close proximity with their target cells via the two reactive portions of the antibody (e.g. the antigen-recognizing domain and the Fc domain), thereby enhancing the efficiency of the treatment.
To date, no NK cell specific bispecific antibodies have been developed. The depleting agents that recruit NK cytotoxicity such as anti-tumor antibodies are typically full-length IgG1 that mediate ADCC via CD16. Despite the existence of a variety of formats for bispecific antibodies, there remains a need in the art for proteins with new and well-defined mechanisms of action that can provide benefits over and can be used in addition to full-length antibodies.